How long, ideally, is a play? The question is palpably absurd. How long is a piece of string? Samuel Beckett's Breath lasts 40 seconds; Eugene O'Neill's Mourning Becomes Electra runs four-and-a-half hours. Form, as in architecture, follows function.

But, while I've no wish to lay down laws, I find myself increasingly disturbed by the fashionable tyranny of the 90-minute play. It is everywhere; and I believe it is crippling ambition, ironing out contradiction, and effectively de-politicising drama.

What are the reasons for the 90-minute rule? I suspect there are several. Social customs are changing and, in a busy restaurant culture, 90 minutes is the ideal postscript to a night out: after Art or Oleanna you ate and argued about the show. Audiences, trained on TV advertising, are also quick to absorb information and no longer need lengthy exposition. There is also the visible influence of the "Edinburgh factor": the bustling hypermarket of the Fringe, where people rush from show to show and anything much over an hour is regarded as an impertinent incursion into one's time.

On the plus side, modern drama has in many cases proved the power of brevity. Beckett's Footfalls uses the hypnotic image of a woman's solitary pacing to externalise her inner anguish. Harold Pinter's Ashes to Ashes shows how a drawing room confrontation can open up to admit the Holocaust. Caryl Churchill's A Number brilliantly explored, in one hour, not only the anguish of parent-child relationships but the defining marks of human identity.

But it is worth remarking that Beckett, Pinter and Churchill all began by writing conventionally structured plays and only gradually mastered the technique of creating images that distill a wealth of human experience. Only a fool would deny dramatists the right to choose the appropriate form. But what worries me is the way relatively young writers are settling into the 90-minute groove: a form midway between pure Beckettian cystallisation of an idea and the once-familiar two-act structure. And while much may be gained - not least for critics up against a deadline - something vital is being lost: the ability to explore the ramifications of a situation or the inconsistencies of human character.

The symptoms are everywhere; but it is particularly striking that the Royal Court, still the epicentre of new writing, has offered us a succession of interval-free plays over the past six months. One result is that you get cut-to-the-chase crisis without social analysis. Joe Penhall, as we know from Some Voices and Blue/ Orange, is a talented writer. But his latest piece, Dumb Show, dealt with the process of celebrity entrapment by tabloid journalists without exploring the wider issues. Who creates the ethos that makes such entrapment possible? Is it an editorial vendetta? Is it a by-product of the circulation war? Or does it spring from some public need to see our secular idols mocked and humiliated? I ended up the none the wiser; and that was because Penhall, in his 90 minutes, had no room to range beyond an examination of a sleazy journalistic device.

The new compressionism can also leave too much unsaid. Kevin Elyot's 70-minute Royal Court play, Forty Winks, was both fascinating and cryptic. You could understand Elyot's theme: the destructiveness of erotic obsession. But, in exploring his peripatetic hero's lifelong preoccupation with his first love, Elyot reduced the other characters to unexplained ciphers: the neurotic love-object, her brutal husband, her narcoleptic daughter. No one is asking for easy resolutions: incompleteness, as Dilys Powell wrote many years ago of Antonioni's L'Avventura, is often part of a work of art's mystery. But Elyot's abbreviated form left the audience not merely to join up the dots but to find where they were located.

Perhaps the gravest charge against the new playella is that it fails to allow room for debate, discussion, dialectic. No one could accuse April de Angelis, a geninely inventive socialist feminist, of a lack of ideas in her latest play, Wild East. It positively bulged with issues: corporate responsibility, economic imperialism, environmental rape, gender politics and many more. But the 80-minute length and the job-interview format meant that attitudes were struck without any counter-propositions being offered. And, having accused big companies of manipulating individuals, De Angelis proceeded to do precisely that with her own characters.

None of these plays was empty or dull; but in each case I felt the dramatist was constricted by his or her chosen form. And, although I've singled out the Royal Court, I could make the same charges against other venues. Recently, for instance, I saw a promising play, Gerald Murphy's Take Me Away at the Bush, about the breakdown of a dysfunctional Irish family. At Edinburgh it had been extravagantly praised. But yet again I felt Murphy had failed to address the really hard question: what is it about the supposedly thriving Celtic Tiger that produces so much misery? In the 1920s Sean O'Casey plotted the connection between poverty and tragedy, so why today has the Irish boom led to noisy desperation?

I am not asking for a standard structure or a return to the days of the two-interval play. But what I miss is the polyphonic richness of which drama is capable, or the complexities of character revealed by an unfolding narrative. One reason why people are flocking to Don Carlos is that it provides exactly the kind of stimulus so much modern drama lacks: exploration of ideas through character, examination of the manifold selves that make up individuals, the thrilling collision of private and public worlds.

You can't, of course, simply re-create old forms: as Alain Robbe-Grillet shrewdly pointed out, Hamlet would not be a masterpiece if it were written today since we do not live in the age of the five-act tragedy. But the new, slavish obeisance to the 90-minute rule stems, I suspect, from a mixture of fashion and ignorance; in particular, a shocking unawareness of even the recent past when drama moved beyond a single situation or point of crisis to examine causes as well as effects.

To put it bluntly, perhaps our own practitioners should simply read more plays. Whatever the remedy, I am getting impatient with these dramatic driblets that offer ideas for plays rather than plays of ideas. Too many of our best playwrights are being inhibited by their surrender to a modish, audience-friendly form.